The dispute is over proposals for conductors to be given a lesser role on a new fleet of Hitachi-built trains due to start running on routes across the Central Belt from next year, including the main Edinburgh-Glasgow line.

Drivers would take over door control and safety duties, leaving conductors to check tickets. On some services, conductors would be replaced by lower-paid ticket examiners.

The plans were sent to staff but withdrawn before being put formally to unions. However, the RMT said ScotRail’s intentions were clear.

RMT general secretary Mick Cash said: “This ballot has demonstrated in the clearest possible way the strength of feeling across the ScotRail network over the threatened extension of driver-only operation.

“The workforce also know only too well that there is a very real threat to passengers of watering down and wiping out the safety-critical role of the guard on these ScotRail services. That is a lethal gamble with basic rail safety. The rock-solid mandate for action will now be considered by RMT’s executive. The union remains available for further talks.”

“Nearly half of our total number of conductors voted against strike action or abstained. This is now a clear signal for the RMT that our people in Scotland want to decide their own way forward.

“We have already guaranteed job security, pay and the futures of our conductors. The RMT must now allow their local representatives in Scotland to consider ScotRail’s ideas that will make these guarantees possible.”